The growth and differentiation (structural, cellular and enzymatic) of intestine will be compared in normally developing rats and in rats in which the physiological milieu has been altered during ontogeny. Fetal, newborn, neonatal and suckling rats will be treated with hormones, and their effects on cell differentiation, cell migration, crypt and villus dimensions and shape and the quantitative pattern of enzymes in their intestines will be compared at several time intervals after the hormone injections to determine both the short-term and long-term effects of hormones on development. The effects of hormones on the enzymatic differentiation in intestines and livers will be compared. Cortisol, thyroxine, glucagon and insulin will be administered individually and sequentially to determine the hierarchy of endocrine stimuli which control intestinal development at different times of life. Fetal rats will be delivered prematurely and postmaturely and will be raised on normal, nutrient or hormone supplemented or deficient diets beginning at birth and on artificial weaning diets beginning on the tenth day. The effects on fetal intestinal development of maternal malnutrition or dietary imbalances will be assessed as will the effects of undernourishment and dietary imbalance of sucklings throughout the postnatal, preweaning period. Hormonal and dietary regulation of adaptations to extra-uterinization and weaning will be emphasized. The interdependence of diet and hormonal effectiveness in postnatal rats will be investigated and the nutrients required for hormones to trigger mature gene expression will be identified.